An ABS for two brake circuits is described in the referenced German patent publication, DE-OS No. 24 33 092, in which a valve arrangement is disclosed and in which, upon a tendency of a wheel, or wheels, to block, pressure is suitably controlled in a specified manner. In particular, the braking circuits are so arranged that diagonally positioned wheels are associated with any one circuit. For example, the left front wheel and the right rear wheel are associated with a circuit I, and then the right front wheel and the left rear wheel are associated with a circuit II. If a tendency of a wheel having a brake in the brake circuit I to block is sensed, a slave control is provided which controls the braking pressure in the circuit II to be somewhat less than that of the braking circuit I. The system, additionally, permits alternate operation under "select low" and "select high" conditions, with respect to sensors which are used in the system, in other words, the sensitivity or degree of braking pressure control can be selected either automatically or under operator control.
Brake control systems are subject to severe requirements. The vehicle must, even under operation of the system, retain its operating stability, must be steerable, and, additionally, the braking distance should be a minimum. Such systems may become complex. Continued developments of such systems are intended to decrease the costs and the complexity thereof, without substantially impairing the effectiveness, although minor refinements of the characteristics may have to be sacrificed, particularly if made to simplification can be obtained thereby. One such system which includes simplification is described in the referenced patent document.
Systems have also been described in which diagonally arranged braking systems have separate control channels for each front wheel, and controlled braking pressure of a front wheel is then applied to the diagonally located rear wheel via a control element which is at least in part responsive to loading. Such systems do not entirely utilize the possible braking effect or braking effort of the rear wheels on the rear axle, particularly if the front wheel is also subject to engine braking, for example if the vehicle has front wheel drive. The vehicle operating characteristics may become critical if the optimum association of braking efforts in the front wheel brakes and rear wheel brakes is changed; this may be due to uncontrollable road surface conditions, inclination of the road surface or the like. The system described in the referenced German Patent publication document also may be subject to difficult operating effects under certain operating conditions of the vehicle.